Depigmenting polymeric compositions with aldehyde settling aids



3,070 553 DEPIGMENTING POLYMERIC COMPGSITIONS WITH ALDEHYDE SETTLINGAIDS Floyd B. Nagle and Norman R. Railing, Midland, Mich.,

assignors to The Dow Chemical Company, Midland,

Mich, a corporation of Delaware No Drawing. Filed July 15, 1958, er. No.748,590

12 Claims. (Cl. 260-23) This invention relates to an improved method fordepigmenting polymeric compositions in order to reclaim the polymericconstituent therefrom for subsequent reuse in place of or in combinationwith virgin polymer stock.

It is a common practice to pigment polymer compositions for coloration,delustering and various other purposes. Among the more popular inorganicpigments for such purpose, for example, are cadmium and seleniumcompounds, titania, the several chromium oxides and the like. In orderto reclaim and recover the polymer constituent from pigmented polymercompositions, it is usually necessary to dissolve the composition in asuitable solvent therefor and to filter the resulting solution ree fromthe insoluble pigment contained therein. This may oftentimes be adifiicult and tedious process and one that may frequently be renderedeven more painstaking by the impossibility of rapidly filtering thesolution, due to the tendency of many pigments to remain suspended inand to settle very slowly from the solution that is made of thepigmented polymeric composition (usually scrap material) beingreclaimed.

It is the principal object of the present invention to provide animproved method for depigmenting polymer compositions to facilitate theeasier and more economically attractive reclamation and recovery of thepolymer constituent thereof. Other objects, advantages and benefits ofthe invention will be manifest in the ensuing description andspecification.

To this end, polymeric compositions pigmented with inorganic pigmentsmay advantageously be depigmented by practice of a method in accordancewith the method of the present invention which comprises dissolving apigmented polymeric composition that is desired to be reclaimed in asuitable non-reactive, water-insoluble, organic solvent thatbeneficially, if possible, may be a hydrocarbon or closely-relatedsolvent; intimately mixing the resulting organic solution containing thedispersed pigment with an aqueous phase that is adapted to acceleratepigment settling and which contains dissolved therein a water-solublechemical settling aid that is non-reactive with the polymer and which,with advantage, is a water-soluble aliphatic aldehyde that contains lessthan about 18 carbon atoms in its molecule; and then separating theorganic polymer solution from the pigment and the aqueous phase,advantageously by settling the organic polymer solution from the aqueousphase; separating the organic polymeric solution from the aqueous phase;filtering the separated polymeric solution; and subsequently isolatingand recovering the reclaimed and depigmented polymer.

' In many cases it is extremely advantageous to incorporate a minorproportion of a Water-soluble cellulose ether derivative, such as methylcellulose and the like, in the aqueous phase before its intimate mixturewith the organic polymer solution. This facilitates flocculation of thepigment in such a manner that the major pro-, portion of the precipitatesettles out in a clean and direct manner, as at the liquid interface oron the bottom of the container in which the depigmentation is beingconducted (when sedimentation techniques are being used for separation)without tending to accumulate on the side walls of the container or atany other point outside of that substantially encompassed by thestratified aqueous phase I ICC layer. By such means, decantation of theorganic polymer solution in order to separate it from the aqueous phaseafter the mixing and settling of the two is made much easier andappreciably more eflicient. It also facilitates complete isolation ofthe polymer solution by filtration or centrifugation procedures forpigment separation, when performed either prior to or after theseparation of the liquid phases.

Practice of the method of the present invention conveniently permits theeffective and efiicient depigmentation and reclamation of scrap polymer.The reclaimed product is generally obtained in a condition of purity andfreedom from contamination by pigment materials that is equivalent tothat of virgin stock with the desired result being achieved with utmostexpedience and minimized operational difiiculty.

Any synthetic, thermoplastic polymeric materialcan be reclaimed fromscrap or other pigmented compositions by the procedure of the presentinvention. For example, various polyam ide (such as nylon 66 or nylon6), polyester (such as polyethylene terephthalate and the like)acrylonitrile polymer (including polyacrylonitrile and other of the wellknown fiber-forming acrylonitrile polymers), haloethylene polymer(including vinyl chloride and saran resins), hydrocarbon polyolefin(including polyethylene and polypropylene) and the like ther moplasticresinous polymers that are capable of being dissolved in suitableorganic, preferably hydrocarbon, water-insoluble solvent media can bedepigmented in this manner. Advantageously, however, the invention maybe practiced with various vinyl aromatic resins which, as the term isconventionally understood, includes the normally solid polymers of suchvinyl aromatic compounds :as styrene, the several chlorostyrenes, theseveral methyl styrenes (including vinyl toluene), the several ethylstyrenes, vinyl naphthalene and the like (particularly those containingaromatic units of from 6 to about 10 carbon atoms, including alkylsubstituents), as well as the solid resinous copolymers of suchcompounds with one another and with other unsaturated monomericmaterials including acrylonitrile, methyl methacrylate, ethyl 'acrylate,a-methyl styrene and other monoethylenically unsaturated monomers anddivinyl benzene, diallyl maleate and the like. v

Any non-reactive, water-insoluble, organicsolvent liquid may be employedin the practice of the invention for dissolving the scrap polymercomposition that is to be depigmented and reclaimed. Obviously, theparticu-' lar solvent that is used depends on the particular poly merthat is involved. For styrene polymers, including polystyrene, it isgenerally suitable to employ ethyl ben zene, xylene, toluene, and thelike or an equivalent water-insoluble aromatic hydrocarbon solvent amongthe known water-insoluble for such polymers. When certain copolymericstyrene polymers, such as copolymers of styrene and acrylonitrile, areinvolved, it may sometimes be preferable to employ relatively more polarsolvents, such as methyl ethyl ketone and the like for their dissolution. Although it is preferred, as a matter of expedience, toutilize a solvent that is practically effective at normal roomtemperatures and under normal atmospheric pressure, it is possible toaccomplish the dis solution andit is generally advantageous to conductthe method of the invention at elevated temperatures. It'is alsopossible to employ superatmospheric pressures in combination with anelevated dissolving and treating temperature. In most cases, however, itmay be found better to avoid greater than atmospheric pressures in viewof the operational difiiculties that may thusbe involved. As isapparent, any concentration of the scrap polymer may be made in thesolution that is to be depigmented. Ordinarily, if possible, it isadvantageous for at least about percent and preferably at least aboutpercent by weight of the pigmented polymer to be dissolved in theorganic solvent. This avoids the inconvenience of handling larger thannecessary volumes of the desired polymer in solution while permitting asuitable settling effect to be accomplished during the depigmentationprocess.

The amount of the aldehyde settling aid that may be incorporated in theaqueous phase with which the dissolved polymer solution is treated maybe found to vary over a wide range depending upon the characteristics ofthe particular settling agent that is utilized when one is beingemployed and upon the nature and quantity of the pigment that is beingremoved from the polymer composition being reclaimed. It is ordinarilybeneficial when it is possible to doso for the aqueous phase to containbetween about 0.5 and 50 percent by weight based on the weight of theaqueous solution of the chemical settling aid that is employed. It isgenerally more practical for the aqueous phase to contain at least about10 percent by weight of the chemical settling aid. The precise amount ofthe settling aid that is employed may, of course, vary with theparticular characteristics of the material and the volume of the waterthat is actually used in the aqueous phase. As a safe rule of thumb, itis usually suitable to employ a quantity of the settling aid to bedissolved in the aqueous phase for each treatment of the organic polymersolution that is about one-half to fifty times by weight the weight ofthe pigment being removed from the polymer composition. In many cases itmay be suitable and preferred for an amount of settling aid to beemployed that is not more than twenty times the weight of the pigment.

The relative quantity of the organic polymer solution and the aqueoustreating phase may also vary over wide ranges. As little, for example,as about 1 to as much as about 9 parts by weight or more of aqueousphase for each part by weight of polymeric solution that is beingtreated may be satisfactorily employed. It is generally preferred forthe weight ratio of the organic polymer solution to the aqueous treatingphase to be about two or three to one, respectively.

Typical of the aldehyde chemical settling aids that may be utilized areformaldehyde and the like including alpha hydroxy adipaldehyde;2-furaldehyde; pyruvic aldehyde; glutaraldehyde; and succinaldehyde.

Besides methyl cellulose, other Water-soluble cellulose etherderivatives may also be incorporated with advantage in the aqueous phasein order to obtain better precipitation of the pigment. as carboxymethylcellulose, hydroxypropyl methyl cellulose and the like which may beemployed in viscosity grades between about 10 and 2,000, preferablyabout five hundred, centipoises as determined by measurement of theirtwo percent aqueous solutions at room temperature. It is ordinarilysatisfactory to employ between about 0.10 and 1 percent by weight of thecellulose ether derivative in the aqueous phase, based on the weight ofthe aqueous solution. It is usually advantageous to employ between about0.10 and 0.25 percent by weight of the cellulose derivative.

, The intimate mixture of the phases during the actual depigmentationinterval may be accomplished by any conventional mixing or physicalagitating technique using ordinary apparatus for such purpose. Usually,at room temperatures, ecah treatment may be accomplished within an hourbefore separating the phases. When elevated temperatures are employedduring the intermixing of the phases, shorter periods of time (as, forexample, 10 minutes or less) may be found to sutfice. After settling,the organic polymeric solution may be separated from the usually heavieraqueous phase in any desired manner that is conventionally employed forsuch purposes, including, for example, sedimentation and decantation andother separatory techniques. After the dissolved polymer These includesuch materials has been filtered, subsequent to its depigmentation, itmay be isolated in any desired manner, including solvent stripping,precipitation in a liquid vehicle followed by filtration and the like.

Alternatively, as has been indicated, the depigmented polymer solutionmay be isolated and separated from the settled pigment by filtering orcentrifuging it prior to the phrase separation of the immiscibleliquids, or subse quent thereto if the settling of the pigment out ofthe organic phase has not gone to completion before their separationupon stratification.

The invention is further illustrated in and by the following examples,wherein unless otherwise indicated, all parts and percentages are to betaken by weight.

Example I About twelve parts of a 38 percent aqueous solution offormaldehyde in distilled water was added to about twenty-eight parts ofa solution containing about 10 percent of polystyrene pigmented withabout 2 percent by weight of titanium dioxide, in toluene. The twophases were shaken together and intimately intermixed for about 10minutes at a temperature of to C. after which they were allowed tosettle. The depigmented organic polymeric solution was then decanted andsubjected to a spectrophotometric light transmission test, using a ModelB Beckman spectrophotometer with light having a wave length of about 55l-L/L, measured through a cell in which the solution was containedhaving a thickness of about 1 centimeter. After about 15 minutessettling, a 3 percent transmission reading was obtained. Thiscorresponded to the reading that was obtained with a standard solutionof the same type in which percent of the pigment had been known to havesettled out. In comparison, the same solution of the pigmented polymerin toluene (without having been treated as above) was found, after 15minutes settling, to have 0 percent light transmission.

Upon recovery of the polymer from the organic solution that had beendepigmented with the aqueous solution of formaldehyde, the polystyrene(which was reclaimed by vacuum devolatilization of the solvent vehiclefor 2 hours at 4 mm. Hg and C.) had properties and color about as goodas virgin polymer styrene.

Example II The procedure of Example I was repeated with about the samerelative quantities of organic solution and different aqueous solutionsof various aldehyde chemical settling aids; one of which was a 50percent aqueous solution of Z-furaldehyde; another of which was a 25percent aqueous solution of alpha hydroxy adipaldehyde; another of whichwas about a 48 percent aqueous solution of pyruvic aldehyde; another ofwhich was about a 26 percent aqueous solution of glutaraldehyde; andanother of which was a 27.6 percent aqueous solution of succinaldehyde.After shaking and settling in the same way, about the same excellentresults as set forth in the first example was obtained with each of thedepigmented organic solutions.

Example 111 The procedure of the first two examples was repeated,excepting to dissolve about 1 percent by weight of methyl cellulose,having a viscosity rating of about 500 centipoises, in each of theaqueous phases. The flocks of pigment which formed deposited directly atthe interface between the aqueous and the organic solutions withoutaccumulating or depositing on the side wall of the vessel. Quantities ofthe cellulose ether as low as 0.1 percent by weight also provided highlysatisfactory facilitation of the flocculation of the settled pigment.

Similar excellent results may be obtained when other aldehyde chemicalsettling aids of the indicated varieties are employed for thedepigmentation and other materials are removed from the otherpolystyrene and analogous polymer compositions of the varietiesmentioned in the foregoing.

The scope and purview of the present invention is to be gauged in thelight of the hereto appended claims rather than strictly from theforegoing illustrative description and specification.

What is claimed is:

1. Method for depigmenting synthetic, thermoplastic polymer compositionswhich comprises dissolving a pigmented polymeric composition containingan inorganic pigment that is insoluble in solvents in which saidpolymeric composition is dissolved in a water-insoluble organic solventfor said polymeric composition that is non-reactive with said polymerand said pigment and in which said pigment is insoluble; intimatelymixing each part by weight of the resulting organic solution containingthe dispersed pigment with between about 1 and 9 parts by weight of anaqueous phase that contains dissolved therein between about 0.5 and 50weight percent, based on the weight of the aqueous phase, of awater-soluble aliphatic aldehyde that contains less than about 18 carbonatoms in its molecule; and then physically separating the organicpolymeric solution from the settled pigment and the aqueous phase.

2. The method of claim 1, wherein between about 0.1 and 1.0 percent byweight of a water-soluble cellulose other is dissolved in said aqueousphase, based on the weight of the resulting solution.

3. The method of claim 2, wherein between about 0.1 and 0.25 percent byweight of the cellulose ether is dissolved in said aqueous phase.

4. The method of claim 2, wherein the cellulose ether derivative ismethyl cellulose.

5. The method of claim 1, wherein the quantity of settling aid that isadded to said solution in an amount that is from about one-half to fiftytimes by weight the quantity of pigment being removed from the polymericcomposition.

6. The method of claim 1, wherein the organic polymer solution containsat least about 5 percent by weight based on the weight of the solution,of the polymeric composition that is to be depigmented.

7. The method of claim 1, wherein between about 10 and percent by weightof the aqueous phase is employed for treating the polymeric solutionthat is being depigmented, based on the total weight of the aqueousphase and the organic polymer solution.

8. The method of claim 1, wherein said synthetic, thermoplastic polymercomposition being depigmented is a styrene polymer composition.

9. The method of claim" 8, wherein the polymer is polystyrene.

10. The method of claim 8, wherein the polymer is polystyrene and thesolvent is ethylbenzene.

11. The method of claim 8, wherein the polymer is polystyrene and thesolvent is toluene.

12. The method of claim 8, wherein the organic polymeric solution andthe aqueous phase are intermixed at an elevated, non-boilingtemperature.

References Cited in the file of this patent UNITED STATES PATENTS2,054,432 Lister Sept. 15, 1936 2,161,949 Calcott et al June 13, 19392,639,278 Stott et a1 May 19, 1953 2,728,725 Gloor Dec. 27, 19552,914,492 Bennett et a1 Nov. 24, 1959 2,915,482 Nagle et al. Dec. 1,1959

1. METHOD FOR DEPIGMENTING SYNETHIC, THERMOPLASTIC POLYMER COMPOSITIONS WHICH COMPRISES DISSOLVING A PIGMENTED POLYMERIC COMPOSITION CONTAINING AN INORGANIC PIGMENT THAT IS INSOLUBLE IN SOLVENTS IN WHICH SAID POLYMERIC COMPOSITION IS DISSOLVED IN A WATER-INSOLUBLE ORGANIC SOLVENT FOR SAID POLYMERIC COMPOSITION THAT IS NON-REACTIVE WITH SAID POLYMER AND SAID PIGMENT AND IN WHICH SAID PIGMENT IS INSOLUBLE; INTIMATELY MIXING EACH PART BY WEIGHT OF THE RESULTING ORGANIC SOLUTION CONTAINING THE DISPERSED PIGMENT WITH BETWEEN ABOUT 1 AND 9 PARTS BY WEIGHT OF AN AQUEOUS PHASE THAT CON TAINS DISSOLVED THEREIN BETWEEN ABOUT 0.5 AND 50 WEIGHT PERCENT , BASED ON THE WEIGHT OF THE AQUEOUS PHASE, OF A WATER-SOLUBLE ALIPHATIC ALDEHYDE THAT CONTAINS LESS THAN ABOUT 18 CARBON ATOMS IN ITS MOLECULE; AND THEN PHYSICALLY SEPARATING THE ORGANIC POLYMERIC SOLUTION FROM THE SETTLED PIGMENT AND THE AQUEOUS PHASE. 